This invention relates to an electroluminescent panel for use in displaying an image, such as alphanumeric symbols, a static picture, a motion picture, and the like, in an input/output device of a computer and so forth.
As a rule, an electroluminescent panel of the type described is known as a display device of a flat type and comprises a transparent substrate, a transparent electrode member on the transparent substrate, a back electrode member opposite to the transparent electrode member, and an electroluminescent layer intermediate between the transparent and the back electrode members. The transparent electrode member is divided into a first set of conductors extended in a predetermined direction while the back electrode member is divided into a second set of conductors extended along another direction intersecting the predetermined direction.
In such an electroluminescent panel, a first dielectric layer is interposed between the transparent electrode member and the electroluminescent layer. In addition, a second dielectric layer is often laid between the electroluminescent layer and the back electrode member. Each of the first and the second dielectric layers should have a high breakdown voltage and a high relative dielectric constant in addition to a low dielectric loss.
In addition, the first dielectric layer should intimately adhere to both the transparent electrode member and the electroluminescent layer and should withstand a heat treatment of a high temperature because the electroluminescent layer must inevitably be subjected to the heat treatment so as to activate the electroluminescent layer. In other words, cracks and peel off must strictly be avoided in the first dielectric layer during and after the heat treatment.
Likewise, the second dielectric layer must tenaciously adhere to both the electroluminescent layer and the back electrode member. Occurrence of cracks and peel off should also be avoided in the second dielectric layer like in the first dielectric layer. Moreover, the second dielectric layer must be resistant to an etchant which is used on forming the second-set conductors of the back electrode member. Otherwise, the etchant might adversely affect the electroluminescent layer underlying the second dielectric layer, as a result of permeation of the etchant into the electroluminescent layer.
In Japanese patent publication No. Syo 58-175,294 (175,294/1983), Y. Fujita et al point out that, when each of the first and the second dielectric layers is deposited by sputtering in the presence of oxygen plasma, the transparent electrode member and the electroluminescent layer are subjected to serious damage by the oxygen plasma during deposition of the first and the second dielectric layers, respectively. Such damage deteriorates characteristics of both the electroluminescent layer and the transparent electrode member. More particularly, the transparent electrode member is unpleasingly reduced in conductivity due to the damage and blackened or darkened due to the damage. The second dielectric layer is peeled off from the electroluminescent layer.
In order to avoid the damage resulting from the oxygen plasma, each of the first and the second dielectric layers is made of a compound of Si.sub.3 N.sub.4 --AlN--Al.sub.2 O.sub.3 --SiO.sub.2, which is referred to as SiAlON. The layer of SiAlON can be deposited by sputtering a target of a mixture of Si.sub.3 N.sub.4 powder and Al.sub.2 O.sub.3 powder in an atmosphere of argon and nitrogen without occurrence of any oxygen plasma.
However, such a compound layer of SiAlON has a low dielectric constant between 6 and 7. Therefore, it is difficult to effectively supply the electroluminescent layer with an a.c. electric field. Inasmuch as the compound layer is complexed in composition, it is difficult to adjust each ingredient to a desired range.